Process for monosulfite recovery



April 23, 1957 w. s. cooK PRocEss RoR MoNosuLFITE RECOVERY 2Sheets-Sheet 1 Filed July 2 9, 1952 www nur

J w mourn! ATTORNEY:

April 23,v 1957 w. s. cooK PROCESS FOR MONOSULITE RECOVERY 2Sheets-Sheet 2 Filed July 29, 1952 INVENTOR Wil/iam .5. Cao/r@ ATTORNEYSUnited States Patent My presentf inventioni` relatesto r an.- improvedmethod of converting sodium'suliide to:sodium".suliite.` More`particularly it relates to an improved methodf of'con-a vertingso-called vsmelt liquor*containing:essentiallyVI sodium sulfide andsodium carbonate to liquor containing essentially .sodiuml suliitesuitable: for cooking wood by the so-called monosultite method..`

Thismethod of pulpinglhas 'long beeni proposed but its use has beenlimited because ofdifculties inthe recovery of sodium and sulfur valuesin the waste liquor. Y

ln most instances the waste liquor has been discarded, thereby creatinga: streamv lpollution problem, or yutilized inflthe recovery system of`a kraftmilljwhere 'such mill;

happens to be convenientlyzadjacent. When'the-fwaste liquor isconcentrated and burned, the sulfur presentis reduced to the sulfide,thereby" posing lthe problemgofrv converting the sulfide tothe swulite.VAMany proposals Ahave been made --to thisend.v These may be divided intoat least `tive general fcategoriesfviz., precipitation of in.. solublefstilides; direct oxidation, .of `the smelt solutions;-electrolysis;.ionexchangeg and, finally, `liberation of the suifurrashydrogen sulfideand its subsequent disposition either by `oxidation orotherwise.

Except yfor some of thetmethods involving electrolysis,

and ion exchange, those ,named require concentration and burning' oftheWaste liquor..

My presentmethod relates to the-last named, i. e., that involving`theliberation andsubsequent oxidation of hydro-v lt also involves aninitial carbonation off-y gen sulfide. the smeltzliquor ,whichroperatesto release .the sulfur, combined v as sodiumsulfitle; as hydrogensulfide., Most of the heretofore proposed methods which involve theformation ofhydrogen sulfiderequire ythe :burning of itto sulfurdioxide. l it will `be observed thatinfcarrying-.out suchy methods anexcessfof ycarbon dioxide must lbe employed in'r order-for it to act asa strippingagentto-carry fromtheu solution the formed hydrogen sulde.-Now whenfthef mixture-of carbondioxideand hydrogen'sulde Ais burned,

aa'gaseousfmixture,is obtained consisting essentiallytof l thecarbondioxide v originally presenu: they lsulfur dioxideq formedzby" theair oxidation of the hydrogen suldeand thev largevolume ofv nitrogenremaining from the air oxidation. Thereby a mixture is had` whichbecauseofl its great volume t frequiresfequipment of l extreme size andof'high` .cost -tohandle it. Furthermore vthe AAcorrosive'i quality ofthe gaseous mixture addszto the difficulty.

Nevertheless for the processto becommercially feasible,;.zthessulfurvalues must be :recovered .to :a substantial de-:A

gree..

My invention is predicated upon myy discovery -thatfv the above type ofrecovery :method may be greatly simpliiied and .improved byestablishing-a closed gas cycle in which a recycled stream of carbondioxide `willperf0rm Patented Apr; n 23, ,1957* ice..

21;' sulfide' with elimination off substantiallyy ,all ofy the sulfidesulfur from ithecarbonate solution;

(3 After the removal of .the hydrogen sulfide', ,.aseleg mental fsulfur;by reactionwith sulfur dioxide,'prefereA ably .in the formofrsulfurous-acid, of actingnas a strip-rl.

ping or desorbing,agenty for a sufcient quantity-ofsulfffur dioxide fromits-solution in water 4whereby the mix-` required.

My improved method involves the following known Advantages which' willbe readily apparent arethat the major equations involved,i. e., Nos. 2and 5, readily l go to completion with the result that noappreciablequantity'of sulfide sulfurescapes reaction and no appreciablequantity of sulfur dioxide is admitted to the carbonating step.v Oifhandit might be supposed that the presence of sulfur'dioxide in-thecarbonating column would merely anticipatel the reaction which iscarried out in the suliting column `and hence would beI withoutsignificance. However, to sullite in the carbonatingcolumn would havethe result of giving riserto large quantities of thiosulfateswhich`represent a complete loss of useful sulfur. Such thiosulfates areformed according to the following reaction which `easily goes tocompletion:

2Na2S+2Na2CO3+4SO2+H20 3Na2S203--2N3HCO3 1 Substantially' nofsodiumsalts are lost and under the conditions obtaining for the oxidation ofthe HzS formed,

. carried outy under atmospheric pressure and with a minimum of suchsteps asevaporatio'n,,precipitation and ltration. Since no airisadmitted to the sulfiting column, there is no tendency to form sulfates.Other advantages will ,be apparent as the description proceeds and thefeatures of novelty4 will be pointed out in the appended claims.

Thepinvention will` be best understood by reference to the followingYdetailed description taken with the annexedv drawingsin `which:l

Fig. l is a process tlow diagram of a preferred embodiment;

Fig.A 2 is a somewhat diagrammatic showing of afplant for carrying outthe process shown in Fig. l.

For sake of brevity, the following lchemical symbols' will be used inlieu of the names of the more frequently referred to chemicalcompoundsinvolved in the process: for carbon'idioxide, CO2; for sulfur dioxide,SO2; for hydrogenksulide, Has; lfor sulfurous acid, H2503; for `sodiumsulde, Na2S;' for sodium sulte, NazSOa; forse dium monocarbonate,NazCOa; for sodium bicarbonate, NaHCOs.

A brief description of the method will iirst be given followed by specicexamples of quantities of materials used, reference being had to Fig. 2.Smelt liquor is introduced through pipe 9 into the carbonating column 10countercurrent to a stream of substantially pure CO2 introduced at thebottom thereof through the pipe 11. The carbonation as represented byEquations 1 and 2 takes place and the excess CO2 sweeps with it theformed H28 out the top of the column through pipe 12 through which it isconveyed to the bottom of column 13 which is the sulfur oxidizing columnand stripper. Normally the HzS formed will be about 10% of the mixtureof CO2 and HzS, i. e., 7 moles of-COz to one mole of HzS. There the HzSmeets the HzSOs introduced into the top thereof through pipe 14 and inthe column the oxidation of the H2S to elemental sulfur (Equation 3)takes place. The slurry of sulfur and water which is formed leaves thebottom of the column through pipe 15 and, aided by pump 16, is passedinto the sulfur settling tank 16a. By having the HzSOa as nearly spentas possible at the bottom of the tower where it meets the incoming CO2and HzS mixture, any polythionic acids formed further up the tower orelsewhere will be largely reduced to sulfur.

The CO2 stream in column 13 desorbs some of the SO2 and this mixture ofCO2 and SO2 leaves the top of the column through pipe 17, and aided bythe blower 18 is introduced into the bottom of sulting column 19 inwhich it meets the sodium bicarbonate solution from column 10 whichilows through pipe 20, pump 21, to the top of said column 19. Thereinthe sodium bicarbonate or monocarbonate present is converted to sodiumsulte substantially in accordance with Equation 5. An agitator 21a incolumn 19 serves the purpose of releasing dissolved CO2 from thesulfited liquor; otherwise the dissolved gas may interfere with thesubsequent pumping operation. The resulting sodium sulte solution is theproduct of the system and leaves the column 19 through the pipe 22. Thesulfur from the settling tank 16a is removed therefrom through pipe 23to the dewatering press 24 and a more concentrated slurry leaves throughpipe 25 by which it is conveyed to the sulfur melt tank 25a and thenceto sulfur burner 26. Make-up sulfur is added to tank 25a through pipe27. The products of the air oxidation of the sul-fur pass through thecolumn 28, the SO2 removed by the action of water from tank 16aintroduced into the column through pipe 29. The HzSOa formed leaves thebottom of column 28 by pipe 30, and aided by pump 31, passes throughpipe 14 to the column 13. The amount of CO2 used will in general bedetermined by that needed in the stripping column 13 since for the mostpart that required for stripping in said column will be little more thantwice that required for carbonation. That needed in the stripping columnis governed by the composition of the equilibrium mixture of the twogases, i. e., of CO2 and SO2, which is dependent on the parti-alpressure of SO2 over the sulfurous acid. In this manner the CO2 requiredis related to the concentration and temperature of the sulfurous acid,regard being had to the fact that as the temperature is raised theequilibrium partial pressure of the SO2 is also raised so that when theSO2 from the sulfur burner 26 is absorbed at the higher temperature aweaker acid is produced; on the other hand, a higher temperature willcause a greater amount of SO2 to be desorbed by the CO2 in the strippingoperation. Lower temperatures are advantageous in avoiding vent losses.

The amount of SO2 desorbed in sulting column 19 will bear asubstantially stoichiometric relationship to the amount of sodium saltsof carbonio acid formed in the carbonating step, it being noted fromEquations and 6 that one mole of SO2 is needed for each two mols ofsodium present regardless of the amount of the carbonic acid radicalpresent. The excess CO2 mayif desired be carried along through thecarbonator provided the column will accept so large a gas ilow.Preferably a valved by-pass 32 is employed for the purpose of bypassinga portion of the CO2. Where the amount of CO2 increases beyond thatneeded as make-up for losses, as may happen through the operation ofEquation 5, the excess is bled olf through valved by-pass 34 todisposition not shown.

yIt will be noted that the concentration of the H2803 is determined bythe fact that it is obtained by passing the products of the -airoxidation of sulfur through a scrubbing tower under atmosphericpressure. Under these conditions, the partial pressure of the SO2 in theresulting solution is such as to limit the amount of dissolved SOz to aValue of from 1 to 2% depending upon the temperature as above pointedout. Greater concentrations than this would result in an undesirablylarge amount of SO2 being stripped from the solution and added to thegas cycle. Furthermore, by circulating a larger -amount of water in theso-called acid cycle (see Fig. 1), the concentration of acid is lowerwhereas if a smaller amount of water is circulated, the concentration ofacid ishigher.

The use of the dewatering press 24 is optional; also it has been foundof advantage to add a occulating agent to ,the sulfur slurry to aid inits precipitation. Of such agents, alum (aluminum sulfate) has been usedsatisfactorily.

Example l In this example the smelt liquor from the pulping of one tonof wood is introduced to the column 10. rlhis liquor has the followingcomposition:

To run this amount of liquor, 2000 lbs. of CO2 is admitted through pipe11. When the reaction is complete, 74 lbs. of H2S is formed and added tothe 2000 lbs. of CO2 leaving the top of the carbonating column throughpipe 12. 332 lbs. of SO2 in 33,600 lbs. of water is introduced into thecolumn 13 (pipe 14), producing 95 lbs. of sulfur by reaction with the 74lbs. of HzS. It may be noted at this point that counter-current ow ofgas to liquor in the column 13 is important in order that, as alreadypointed out, the spent acid may contact the incoming HzS so as tominimize the formation of polythionic acids or to reduce any such thatare formed to sulfur. It is therefore desirable to provide thestoichiometric quantity of SO2 to oxidize the HzS plus what is to bedesorbed by the stream of CO2. In order to provide for 256 lbs. ofSOz tobe desorbed by the CO2, a total quantity of 332 lbs. of SO2 is providedin the 33,600 lbs. of water. This leaves the exiting liquorsubstantially free of sulfurous acid. Some polythionic acids are presentin the liquor and are recirculated.

The lbs. of sulfur is allowed to settle and the thickened slurry passedto the press 24. The sulfur now in the form of a still thicker slurry isconveyed to the melt tank 25a and molten sulfur is passed to the sulfurburner 26 as has already been described and additioned by 60 lbs. ofmake-up sulfur.

672 lbs. of carbonates expressed as sodium bicarbonate is formed in thecolumn 10 and is passed in solution in 10,000 lbs. of water to thesultiting column 19 where it is reacted with 256 lbs. of SO2 alreadymentioned, thereby producing 504 lbs. of sodium monosulte and 352additional pounds of CO2 which most conveniently is vented through pipe34.

Example 2 In this example a liquor low in sodium sulte is used. Itscomposition is:

and producing 15 lbs. of sulfur. The sulfuris recovered" and burned aspreviously described; 1,19 lbs. of makeup sulfur is required, the samebeingl admitted through pipe 27 as heretofore.

Example 3 In this example a smelt liquor richin NazS is usedjwhich hasthe following composition:

Na2S, lbs 311 NazCOs, lbs Water, lbs 10,000

Upon carbonation, 136 lbs. ofi-.H28 is formed in the same 2000 lbs. ofC02, it being noted that more CO2 is consumed because of the greateramount of H2S formed. The carbonates. formed, expressed as NaHCOsareapproximately 672 lbs. as in the preceding examples. In order tooxidize. the 136 lbs. of H28', 140 lbs. 0f S02 is required,sufcientadditionalSOz being desorbed to bring the S02 content of theexiting CO2 to the same value of4 256 lbs. as in the precedingexamples.170 lbs. of sulfur is formed which is fed to the sulfur burner 26 asbefore. 24 lbs. of make-up sulfur is needed in this example.

In the above examples, the temperature within the column is maintainedat the ordinary ambient temperatures, i. e., l0-80 F. for the region ofCharleston, S. C. Elevating the temperature of the liquor duringcarbonation results in a shift of the equilibrium to give lessbicarbonate formation. At about 160 F. a solu tion having a maximum ofabout sodium monosulte is produced. The proportion of the monocarbonateto bicarbonate is not critical provided only that the removal of thesulfide sulfur is complete. However, by thus throwing the equilibrium inthe direction of the monocarbonate formation, a higher concentration ofsmelt liquor may be employed without precipitation of NaHCOs andconsequently a higher concentration of sodium sulte solution may beobtained as the nal product of the system.

If desired, instead of carrying out both the operations of stripping theS02 from the H2803 solution and oxidizing the H2S in the same tower,these operations may be carried out separately. Thus the gaseous mixturefrom the carbonator may be led to a first or oxidizing column and therecontacted with a solution of H2S0a in suicient amount to oxidize the H2Sto elemental sulfur substantially quantitatively and the resultingsulfur slurry passed to a settling tank as heretofore described. Theoxidation of H28 can if desired be effected by use of gaseous SOzinstead of H2803, as is apparent to those skilled in the art. Hence theuse of the expression SO2 as the reagent in the oxidation of HzS toelemental sulfur should be taken to embrace both the gaseous S02 and itsequivalent solution form, H2803. Then the gas stream deprived of its H2Sbut containing some acquired S02 is passed to a second or strippingcolumn to which a further supply of H2503 is admitted and in which therequired amount of S02 is picked up by the gaseous stream. Spent acidfrom the stripping column is passed directly to the SO2 adsorber. By soconducting the oxidizing and stripping operations in two steps insteadof one, the concentration of H2803 may be varied in the two columns bestto suit the conditions there obtaining. Furthermore the sulfur slurryobtained in the oxidizing column will be advantageously somewhat moreconcentrated.

In the foregoing examples the sulfur lost has amounted to approximately15% chiefly as polythionic acids; in larger scale operations it would beexpected that such loss would be considerably less.

Also inlieufof theiratherdilute solution of smelt liquor as set forth inthe above specific examples, a stronger,

smeltsolution may be used in the, initial-carbonationstep, i. e.,onecontaini-ngl in the neighborhood of 20% solids. Then duringl thecarbonation, the relatively insoluble sodium bicarbonate formed willprecipitate out. At the completion ofvvthecarbonation, the precipitateof sodiumbicarbonate may-.beliiltered off and washed, thereby giving asodium bicarbonate of high purity. The mother liquor filtered off maythen be sent back to the evaporators to be added to the concentratewhich is about to be-burnedto form a, new smelt. The recovered sodiumbicarbonate may then be dissolved in suicient water and sulited vin the.manner already described.

Various modifications will occur to those skilled in the .art withinthe. spirit of the present invention. For. example, if, desired thevcarbonating column may be superimposed upon the suliting column in sucha manner that there will be created' a lower sulfiting zonel andan uppercarbonating zone and the same result had.

It will be understood that if desired thesuliiting operation insteadofbeing carried to thecomplete formation of monosulte may be carriedmerely to a partially sulfted stage consistingfofa mixture of-sodiumsuliite andthe sodium salts of carbonic acid, in which case a gaseousmixture weaker in S02 will be used.

Likewise the suliting operation may be carried out to producesodiurnbisuliite bythe use of a gaseous mixture containing a greaterportion of S02. 0r, if desired, a product may be obtained comprising amixture of the monosulfite and bisullite salts. By using even greaterportions of SO2 mixtures of sodium bisulfite and free sulfurous acid canbe produced. By so proceeding the process will be adatped for furnishingcooking liquod suitable for the soda base acid suliite process providedonly that the waste liquor from such process be satisfactorilyconcentrated and burned to produce a smelt liquor.

When wood pulp is manufactured by digesting the woo-d in sulfurous acidwhich is wholly or partially combined with an alkali metal (sodium) andthe .resultant black liquor collected and burned in a smelter furnace, asmelt results which when dissolved in water is termed a smelt liquor.Smelt liquor is frequently called green liquor because impurities areoften present which impart a green color to the solution. The smeltliquor is essentially a solution of sodium sulfide and sodium carlbonate. The present process provides a method whereby the smelt liquorcan be converted to fresh pulping liquor.

I claim:

1. The method of converting sodium sulfide, together with any sodiumcarbonate present in smelt liquor obtained from the pulping of wood, tosodium suliite which comprises passing a stream of C02 into Contact withsaid liquor to carbonate same with the formation of sodium salts ofcarbonio acid and H28 and continuing such passage of such stream untilsuch H28 is stripped substantially completely from said liquor, passingthe H2S bearing stream of C02 to an oxidizing zone wherein said streamis brought into contact with SO2 whereby to oxidize said HzS toelemental sulfur, removing said sulfur from said stream, enrich-ing saidstream of CO2 with SO2, utilizing said S02 enriched Stream to sulte thealready carbonated smelt liquor to convert the carbonates pres.- ent tosuliites and to obtain substantially pure CO2, and utilizing said CO2 so`obtained to carbonate a fresh supply of smelt liquor, the amount of CO2used in the carbonating step being at least tive times thestoichiometric requirement.

2. The method of converting sodium sulfide, together with any sodiumcarbonate present in smelt liquor obtained from the pulping olf wood, tosodium sulte, which comprises passing a stream of C02 in intim-atecontact with said liquor to carbonate same with the formation of sodiumsalts of carbonic acid and H2S and simultaneously to strip from saidliquor said HzS, the quantity of CO2 utilized being in substantialexcess of the stoichio metric iamount for carbonating said liquor toeffect a substantially complete stripping of said H2S from said liquorby CO2 sweeping, passing the Hzs-bearing stream of CO2 to Ian oxidizingzone wherein said stream is brought into contact with SO2 whereby tooxidize said H28 to ielemental sulphur, removing said sulphur from saidstream, enriching said stream of CO2 with SO2, and utilizing saidSO2-enriched stream to sulte 4the already carbonated smelt liquor toconvert the carbonates present to sultes and to obtain substantiallypure CO2.

3. rlhe method as defined in claim 2, wherein said stream of CO2 iscaused to be enriched with SO2 simultaneously with sai-d oxidizing step.

4. The method as dened in claim 2, wherein the oxidation of the H2S isbrought about by contacting the CO2 stream bearing the same with SO2 inaqueous solution in coun-tercurrent manner, whereby said solution of SO2is substantially spent upon initial Contact with said stream.

5. The method as dened in claim 2, where-in the elemental sulfur`obtained and removed in the oxidation of H2S is burned to SO2 and suchSO2 utilized for the oxidation of H28 and enrichment of the CO2 stream.

6.4 The method as defined in claim 2, wherein the amount of SO2incorporated in the stream of CO2 as a result of the enriohment stepbears a substantially s ttoichiometric relationship to the amount ofsodium salts of carbonic acid formed in the carbonating step.

7. The method of converting a solution of sodium sulde to sodium sulte,which comprises passing a stream of CO2 in intimate contact with saidsolution to carbonate same with the formation of sodium salts ofcarbonio acid and H2S and simultaneously to strip from said solutionsaid H2S, the quantity of CO2 utilized being in substantial excess ofthe stoichiometric amount for carbonating said solution to effect asubstantially complete stripping of said H2S from said solution by CO2sweeping, passing the E28-bearing stream of CO2 to an oxidizing zonewherein said stream is brought into contact with SO2 whereby to oxidizesaid H28 to elemental sulphur, removing said sulphur from said stream,enriching said stream of CO2 with SO2, and utilizing said SO2-enrichedstream to isulte the already carbonated solution to convert thecarbonates present to sulfltes and to obtain substantially lpure CO2.

References Cited in the le of this patent UNITED STATES PATENTS1,728,252 Rawling Sept. 17, 1929 1,983,789 Bradley et al Dec. 11, 19342,163,554 Gaither June 20, 1939 2,177,707 Gaither Oct. 31, 19392,496,550 Larson et al. Feb. 7, 1950 2,675,297 Gray et al Apr. 13, 1954

2. THE METHOD OF CONVERTING SODIUM SULFIDE, TOGETHER WITH ANY SODIUMCARBONATE PRESENT IN SMELT LIQUOR OBTAINED FROM THE PULPING OF WOOD, TOSODIUM SULFITE, WHICH COMPRISES PASSING A STREAM OF CO2 IN INTIMATECONTACT WITH SAID LOQUOR OF CARBONATE SAME WITH THE FORMATION OF SODIUMSALTS OF CARBONIC ACID AND H2S AND SIMULTANEOUSLY TO STRIP FROM SAIDLIQUOR SAID H2S, THE QUANTITY OF CO2 UTILIZED BEING SUBSTANTIAL EXCESSOF THE STOICHIOMETRIC AMOUNT FOR CARBONATING SAID LIQUOR TO EFFECT ASUBSTANTIALLY COMPLETE STRIPPING OF SAID H2S FROM SAID LIQUOR BY CO2SWEEPING, PASSING THE H2S-BEARING STREAM OF CO2 TO AN OXIDIZING ZONEWHEREIN SAID STREAM IS BROUGHT INTO CONTACT WITH SO2 WHEREBY TO OXIDIZESAID H2S TO ELEMENTAL SULPHUR, REMOVING SAID SULPHUR FROM AND STREAM,ENRICHING SAID STREAM OF CO2 WITH SO2, AND UTILIZING SAID SO2-ENRICHEDSTREAM TO SULFITE THE ALREADY CARBONATED SMELT LIQUOR TO CONVERT THECARBONATES PRESENT TO SULFITES AND TO OBTAIN SUBSTANTIALLY PURE CO2.